We will investigate the controls for centriole duplication and how prolonged prometaphase blocks daughter cell proliferation. Since mitotic fidelity depends upon the cell containing just two centrosomes, centrosome duplication must be under tight numerical and temporal control. Extra centrosomes (centrosome amplification) at mitosis can lead to unequal chromosome distribution and consequent genomic instability which is a driving force in multi- step carcinogenesis. In Aim 1 we test a model that can explain centrosome amplification after DNA damage, a well established but poorly understood phenomenon. Radiation and radiomimetic drugs are currently used to treat human tumors. Follow up radiation therapy can cause DNA damage in proliferating normal cells in the tumor region - particularly after surgery. In Aim 2 we will investigate if and how geminin plays a role in enforcing the block to centrosome reduplication during S and G2 phases of the cell cycle. The centrosome intrinsic block to reduplication is of critical importance in preventing centrosome amplification. In Aim 3 we test if targeting of Cdk2-cyclin E to the centrosome is required for centrosome duplication or if soluble pools of this kinase complex are sufficient to drive centrosome duplication. This explores the importance of local control of centrosome duplication and provides insight into how zygotes control centrosome duplication despite constitutively high cytoplasmic Cdk2-cyclin E activity. In Aim 4 we will further characterize the basis for our discovery that prolonged prometaphase causes an irreversible block to daughter cell proliferation despite the normal division of the mother cell. This proliferation block can serve to handle mitotic defects due to environmental toxins that are never resolved but allow satisfaction of the mitotic checkpoint and consequent improper completion of mitosis. We will also explore the possibility that chemotherapeutic regimes using microtubule targeting drugs could lead to stem cells withdrawing from the cell cycle thereby compromising tissue regeneration, wound healing, and tissue maintenance.